A male rhesus macaque with a cephalic chamber implant for neurophysiology recording presented with hemiparesis affecting the left thoracic and pelvic limbs at approximately 5 wk after craniotomy surgery. MRI indicated a 1×2-cm ovoid cerebrocortical cystic lesion immediately subjacent to the right hemisphere craniotomy and recording chamber. Transdural aspiration of sterile transudate and resultant decompression resolved the hemiparesis, and follow-up MRI at 1 mo indicated resolution of the lesion. Subsequently, necropsy at study end revealed a cerebrocortical foreign body composed of silicon. The atypically slow cure rate of the lot of silicon used and the unique recording chamber configuration were underlying factors that contributed to the formation of this foreign body. To our... knowledge, this report is the first description of iatrogenic intracerebral foreign body in a macaque.